I. Field of the Invention
The present invention relates generally to the field of cardiac rhythm management and more particularly to a dual chamber cardiac pacemaker incorporating a system for discriminating between pacemaker mediated tachycardia (PMT) and simple upper rate limit Wenckebach behavior due to normal sinus tachycardia. The system may also provide for adjusting the upper rate limit (URL) or maximum tracking rate (MTR) according to the frequency of detected pacemaker Wenckebach events.
II. Related Art
In dual chamber pacemakers, circuitry is provided for both sensing atrial and ventricular depolarization events and for pacing one or both of atrial and ventricular tissue. In a subject with normal sinus node activity and interrupted conduction system, the pacemaker is able to sense an atrial depolarization (P-wave) and thereafter stimulate the ventricle in accordance with an established AV delay interval. This effectively mimics the heart's PR interval. The situation is complicated, however, by the possible occasional occurrence of an interfering retrograde conducted P-wave, possibly the result of a ventricular stimulating pulse, but one which is also sensed by the atrial sensing circuitry. Because the atrial sensing circuitry of the pacemaker cannot tell whether a sensed signal is a normal or retrograde conducted P-wave it will initiate another ventricular stimulation event. This may quickly lead to PMT. To overcome this problem, dual chamber pacemakers are typically programmed to include a post-ventricular atrial refractory period (PVARP) during which atrial events are sensed but ignored. In this manner, if an atrial event occurs during PVARP due to retrograde conduction, an AV interval is not initiated and no ventricular stimulating pulse is generated as a result of the atrial event.
The addition of PVARP does not totally successfully resolve the problem either, however, because in many pacemaker treated patients the condition of the patient is such that the retrograde conduction time varies or fluctuates depending upon physiologic feedback mechanisms. This means that a fixed, programmable PVARP may become relatively too short over time if retrograde conduction time increases and may no longer serve to inhibit PMT. Conversely, if the PVARP is programmed to be too long, this shortens the sensing window and as the pacing rate reaches the maximum atrial tracking rate or MTR set for the pacemaker, some of the desirable P-waves will fall inside the PVARP and be ignored and this will result in an undesirable drop in the ventricular pacing rate. This is known as a two-to-one block. Thus, each time a P-wave falls within PVARP and an AV block occurs for that cardiac cycle it results in a missing cardiac cycle which is undesirable because it causes short-term loss of AV synchrony and the subsequent loss of cardiac output.
Pacemaker Wenckebach is another type of undesirable upper rate limit behavior which negatively affects the patient. In pacemaker Wenckebach, as the atrial rate increases, the AV interval is lengthened so that the ventricular pacing interval does not exceed the MTR. As the atrial rate increases, a P-wave will eventually fall within PVARP and AV block will occur for that cardiac cycle. The successive lengthening of the AV interval leading to a missing cardiac cycle likewise causes short-term loss of AV synchrony and subsequent loss of cardiac output. The detection of pacemaker Wenckebach is important as an indication of a possible need for URL/MTR adjustment.
Pacemakers are implanted to typically operate within a particular beat rate or heart rate (HR) range including a particular URL or MTR which is typically picked on the conservative side by the physician who may have a minimum familiarity with the level of activity reached by the patient. The actual proper HR range for the patient may well extend above that initially programmed. This being the case, the pacer may repeatedly reach the URL or MTR because of naturally occurring sinus rhythms and pacemaker Wenckebach may then occur. (As used herein, the term Wenckebach, unless otherwise stated, refers to a pacemaker Wenckebach epidsode). Because URL or MTR may also be reached due to PMT, and PMT should be stopped as soon as possible, because of the alternate loss of cardiac output, there is a definite need to distinguish between these two phenomena.
Pacemakers have been programmed with a function to determine whether a patient is in a pacemaker mediated tachycardia (PMT) by counting intervals that are atrially sensed and ventricularly paced at the URL. After a prescribed number of successive intervals are perceived to be at the maximum rate, perhaps 16, the pacemaker assumes the existence of PMT and is programmed to extend the next PVARP a sufficient time to break or interrupt the PMT. This PVARP extension is typically about 400-500 ms. An example of this approach is found in Walmsley et al (U.S. Pat. No. 5,674,255) assigned to the same Assignee as the present invention. The contents of that patent are deemed incorporated by reference herein for any purpose. While this solution has been successful in interupting and correcting actual PMT, it is unable to predictably discriminate between URL events which are caused by PMT and which occur simply because the patient exercised to the MTR/URL.
Thus, while the above determination and aleviation of PMT has been quite successful, a need remains for adding a technique which would produce a better and more sophisticated analysis of the nature of the cause of the upper rate limit behavior. In this manner, if one could reliably determine whether the rhythm is a PMT or a Wenckebach episode due to normal sinus tachycardia, after a given number of Wenckebach events within a prescribed time, the URL or MTR could be adjusted upward so that the pacemaker range would self-adjust to be more in line with the actual activity level of the patient.
Further, in describing the related art and features of the present invention, it is believed that it would be helpful to define certain terminology. Accordingly, several definitions are presented.
Maximum Tracking Rate (MTR) or Upper Rate Limit (URL) is the maximum rate at which the paced ventricular rate will track sensed atrial events. It is applicable to the atrial synchronous pacing modes, DDD, DDDR, VVDR and VDD and is programmable quantity typically residing in the range of from about 50 to 185 pulses per minute.
AV Delay (AV) is the programmable time period from the occurrence of an atrial event, either sensed or paced, to a paced ventricular event. It is a programmable quantity typically ranging between 10 and 300 milliseconds and is active in DDD, DDI, DVI, DOO, VDD and the similar rate responsive modes.
Post Ventricular Atrial Refractory Period (PVARP) is defined as the time period after a ventricular event, either paced or sensed, during which activity in the atrium does not inhibit an atrial stimulation pulse nor trigger a ventricular stimulating pulse. It is designed to avoid atrial sensing of retrograde activity initiated in the ventricle.
VA Interval is defined as the time period from the occurrence of a ventricular event, either paced or sensed, to the occurrence of an atrial event, either sensed or paced.
Pacemaker Mediated Tachycardia (PMT). In DDD(R) and VDD(R) pacing modes, the pacemaker may detect retrograde conduction in the atrium, causing triggered ventricular pacing rates as high as the MTR. This is referred to in the literature as pacemaker-mediated tachycardia or endless loop tachycardia.
Total Atrial Refractory Period (TARP) is defined as the sum of the AV delay and PVARP.